Clinical and morphologic features of 34 patients with hypertrophic cardiomyopathy and particularly marked left ventricular hypertrophy were analyzed. Only patients with a ventricular septal thickness of at least 35 mm (range to 52 mm) were selected for the study; 31 (90%) had a diffuse pattern of hypertrophy also involving substantial portions of the left ventricular free wall. Ten patients (29%) had hemodynamic or echocardiographic evidence of basal subaortic obstruction (average gradient, 63 mm Hg); however, the majority (24 (71%)) had no evidence of obstruction at rest, despite substantial hypertrophy of the basal anterior portions of septum and free wall. The clinical course was variable in 30 patients who were followed up for at least 1 year (mean 6 years). Although no patient died, nine (30%) have exhibited clinical deterioration, including two who spontaneously developed complete heart block and one who collapsed with ventricular fibrillation but survived. However, the clinical condition of the majority of patients (21 (70%)) remained unchanged or improved. At the most recent evaluation, 20 (67%) of the 30 patients were asymptomatic or only mildly symptomatic, including 7 who remained without symptoms throughout the period of follow-up. The subset of patients described in this report shows the most striking morphologic alterations that occur in hypertrophic cardiomyopathy. Although such extreme increases in left ventricular mass might intuitively suggest a unique clinical expression and course for these patients, the patients nevertheless demonstrated a variety of clinical manifestations; their natural history did not reflect a uniformly poor prognosis over the period of follow-up, and two-thirds of the patients had only mild or no symptoms at the most recent evaluation.